nixos: Shuffle definitions and deprecate system.extraSystemBuilderCmds

Shuffle:
The definitions are now combined into a single option. Since they have
no interdependencies, that's ok, but you may notice this trivial change
by a changed hash, and analyzing with nix-diff.

Deprecation:
Use the option `system.systemBuilderCommands` instead.

nixos/tests/switch-test.nix

@@ -64,7 +64,7 @@ in
         boot.loader.grub.enable = false;
 
         specialisation = rec {
-          brokenInitInterface.configuration.config.system.extraSystemBuilderCmds = ''
+          brokenInitInterface.configuration.config.system.systemBuilderCommands = ''
             echo "systemd 0" > $out/init-interface-version
           '';